ENGLISH ABSTRACT: Lucerne is the most important pasture and fodder crop in the winter rainfall area of
South Africa. Various pests are known to cause damage to this crop. The use of the
sweep net for monitoring pests is a cheap, easy and quick technique. If the sweep net
is suitable for the lucerne pests in South Africa, potential pest status can be
determined easily and quickly and the necessary precautionary measures taken to
prevent crop losses. From a managerial point of view, it is also important to know
the composition of the insect community in order to follow practices in which the
number of beneficial insects can be increased and the injurious insects decreased.
Therefore a study was done to quantify the use of the sweep net as a survey
technique for monitoring pests on established lucerne stands. Insect diversity was
also determined to obtain information on the insect families and guilds on lucerne.
The redlegged earth mite, due to its importance as a pest, and the Anystis mite,
important as a predator, were also included.
The sweep net proved to be suitable for the sampling of the main lucerne pests. If a
29 cm diameter sweep net is swiped once per pace for six long paces, twelve
systematically chosen sampling units are recommended for the lucerne earth flea and
aphids. It is not necessary to differentiate amongst the three aphid species, or
between the winged and unwinged aphids. Actual counts should be used instead of
absence-presence data. Instead of counting all the insects in a sample, sub-samples
can be taken. Operational characteristic curves can be used to determine the risk
involved in the decision not to intervene, for example by spraying or grazing.
Recommendations for monitoring and the accuracy of control decisions for the
redlegged earth mite, Sitona weevil and lucerne butterfly can only be made after
threshold values have been determined.
The pea aphid, bluegreen aphid and lucerne earth flea showed peaks in their
population levels during spring. Peak numbers of the spotted alfalfa aphid occurred
during late summer and autumn. The Sitona weevil and lucerne butterfly numbers
reached peak levels during late spring and early summer. For all pests population levels were dramatically reduced after grazing or cutting of the plantings. Therefore,
these cultivation practices provided good control.
The herbivores made up more than 85% of the insect community in lucerne. The
largest herbivorous families, in terms of the number of individuals per family, were
the Aphididae and Sminthuridae. These two families contain the main lucerne pests,
the pea aphid, bluegreen aphid, spotted alfalfa aphid and the lucerne earth flea. The
largest predatory family was the Anystidae, represented by the Anystis mite, the
most important predator of the red legged earth mite and lucerne earth flea. Another
well represented predatory family was the Coccinellidae, containing natural enemies
of the aphids. The dryland plantings had a higher percentage of predators than the
irrigated lucerne.
The most important parasitaids were those in the superfamily Chalcidoidea and in
the family Braconidae. The main detritivores were springtails in the suborder
Arthropleona, insects in the families Mycetophilidae on irrigated lucerne, and
Mycetophagidae on dryland lucerne. The most abundant visitors were in the families
Chironomidae, Drosophilidae and Tephritidae. The dryland plantings had a lower
percentage of visitors than the irrigated plantings.
The number of insect families, as well as the number of individuals per family, was
lower at the dryland plantings than at the irrigated plantings. The vast majority of
insect families found on lucerne were collected during the one-year sampling period.
A lower diversity was found where grazing was more severe, and there was a
negative relationship between diversity and evenness.